Known in the art is a heat-insulating material comprising from 85 to 98 parts by weight of mineral fiber and from 15 to 2 parts by weight of a binder applied onto said fiber, which is a modified resol phenol-formaldehyde resin on the basis of phenol, a 50% aqueous solution of formaldehyde, barium hydroxide, dicyandiamide, water, and sulphuric acid. For preparing the binder, the following ratio of the components is taken (in parts by weight):
______________________________________ phenol -100 50% aqueous solution of formaldehyde -154 barium hydroxide -15 dicyandiamide -35 water -24.1 Sulphuric acid up to a certain ph. ______________________________________
The heat-insulating material including mineral fiber and a binder of the above composition possesses improved adhesive properties as compared to a heat-insulating material containing sodium hydroxide in the binder composition, but the binder stability deteriorates.
The life of the binder, which is a component of the aforementioned heat-insulating material at a temperature of +5.degree. C. (special thermostated tanks are required) is 4 weeks.
As the binder does not contain any waterproofing additives, the heat-insulating material on its basis does not have water repellent properties.
Also known in the art is a heat-insulating material comprising mineral fiber and a binder applied onto the fiber, which is a modified resol phenol-formaldehyde resion on the basis of phenol, formaldehyde, barium hydroxide, water, urea or melamine.
There is also known a heat-insulating material comprising mineral fiber and a binder applied onto the said fiber, which is a modified resol phenol-formaldehyde resin on the basis of phenol, a 37% aqueous solution of formaldehyde, dicyandiamide, sodium hydroxide, boric acid and a waterproofing additive .gamma.-aminopropyltriethoxysilane (Silane "A-1100").
For preparing the binder which is a component of the heat-insulating material, the following components are taken, in parts by weight:
______________________________________ phenol -100 37% aqueous solution of formaldehyde -340 dicyandiamide -40 sodium hydroxide -3.7 silane "A-1100" -0.001-2 wt. % of the result- ing phenol-formaldehyde resin weight boric acid up to pH = 7.5-8.0 ______________________________________
A modified resol phenol-formaldehyde resin on the basis of phenol, a 37% aqueous solution of formaldehyde, dicyandiamide, sodium hydroxide, silane "A-1100" and boric acid are prepared in the following manner.
A reactor provided with an agitator, a return condenser and a heater element is charged with 100 parts by weight of phenol, 340 parts by weight of a 37% aqueous solution of formaldehyde, and 3.7 parts by weight of sodium hydroxide. pH of the solution in the reactor should lie within the range of 8.5 to 9.0.
The process is conducted at a temperature of 70.degree. C. till the concentration of free formaldehyde in the reaction medium drops below 12% by weight.
The mixture is cooled to a temperature of 40.degree. to 50.degree. C. and the medium is neutralized with boric acid to pH=7.5-8.0. Then, 40 parts by weight of dicyandiamide are added to the reactor, and the mixture is heated to a temperature of 60.degree. to 70.degree. C. Immediately after dissolution of dicyandiamide, the resulting modified resol phenol-formaldehyde resin is cooled to a temperature of 40.degree. C. and from 0.001 to 2% by weight of silane "A-1100" are added thereto.
A disadvantage of the above heat-insulating material is the complexity of synthesis and the high cost of the waterproofing additive, namely, .gamma.-aminopropyltriethoxysilane (A-1100), used in the binder. Besides, it is known that silane "A-1100" is an effective waterproofing agent only for mineral fibers, having a high acidity index (M.sub.a =2.28-2.41).